An exhaust gas exhausted from a chemical plant may contain one or more combustible organic compounds such as saturated hydrocarbons, unsaturated hydrocarbons, organic acids, esters, and alcohols. In view of environmental concerns, it is not preferred to diffuse combustible organic compounds into the air, and therefore, it is necessary that the exhaust gas be purified before it is exhausted out of the chemical plant.
A treatment using a catalytic combustion reaction is proposed as one treatment for purifying a gas to be treated containing a combustible organic compound and exhausting the purified gas out of the chemical plant. It is known that platinum group metal-supported catalysts such as platinum and palladium exhibit high activity as catalysts for combustion treatment. For example, Japanese Unexamined Patent Application, First Publication No. 51-106691 discloses that a catalyst in which platinum and palladium are supported on an alumina carrier is effectively used as a catalyst for purifying an exhaust gas. Furthermore, Japanese Unexamined Patent Application, First Publication No. 2000-33266 discloses that a tin oxide on which palladium, or palladium and platinum are supported is effectively used as a catalyst for purifying an exhaust gas.
However, it is known that catalytic activity of the platinum group metal-supported catalysts gradually decreases due to catalytic poisoning, that is, adhesion of a catalytic poison to an active site of the catalysts, or heat deterioration by sintering or the like. When the catalytic activity is decreased, a method of raising the reaction temperature to maintain purifying capacity is usually adopted.
The amount of the combustible organic compound can be sufficiently reduced by raising the reaction temperature even if the catalytic activity is decreased; but on the other hand, the temperature increase accelerates heat degradation of the catalyst, and then the catalyst life may decrease. If the reaction continues using the degraded catalyst, the amount of the combustible organic compound is not efficiently reduced, and then, gas still comprising the combustible organic compound may be exhausted into the air. In a worst case scenario, the plant inevitably must be stopped, and then an owner of the plant sustains great damage. Therefore, in order to extend the catalyst life, the catalytic combustion reaction is preferably carried out at the lowest reaction temperature at which the amount of the combustible organic compound can be sufficiently reduced.
As a method for ascertaining whether the combustible organic compound has been sufficiently reduced, a gas after reaction may be analyzed by gas chromatography. Gas chromatography can be used to ascertain whether the combustible organic compound has been sufficiently reduced; however, it is not suitable for optimization of the reaction temperature.
For example, if the reaction temperature is changed and gas analysis is carried out after the reaction at each reaction temperature, it is theoretically possible to optimize the reaction temperature by determining the lowest reaction temperature which can sufficiently reduce the amount of the combustible organic compound with the catalytic performance available at the time. However, it requires much labor to optimize a reaction temperature by analyzing in detail by gas chromatography whenever the reaction conditions of the catalytic activity are changed, and it is far from realistic.